That's the finding from a new study of more than 900 adults with potentially life-threatening allergies. The researchers said 52 percent didn't use their prescribed auto-injectors in an allergic reaction emergency.

While 89 percent of people surveyed did fill their prescription for the auto-injector, "almost half (45 percent) said they didn't have [one] with them during their most severe allergic reaction," said study lead author Christopher Warren, of the University of Southern California.

"This was despite the fact that 78 percent of the people responding had been hospitalized for their allergy at some point in their lifetime," Warren said in a news release from the American College of Allergy, Asthma and Immunology (ACAAI).

Another 21 percent said they didn't know how to use their auto-injector, he noted.

About half the survey participants said an auto-injector was accessible (within 5 minutes) all of the time, 44 percent said they carried at least one all of the time, while less than 25 percent said they routinely carried more than one.

Anyone who's been prescribed an epinephrine auto-injector should always have it with them and should always carry two in case a severe allergic reaction recurs, according to the ACAAI.

Also, epinephrine should be administered at the first sign of a severe allergic reaction, particularly if a person has had a previous severe allergic reaction or has both a severe allergy and asthma.

But doctors who see many cases of severe allergic reactions agreed that too many people remain unprepared.

"There are many barriers to the optimal level of having everyone who may need it carry the epinephrine all the time, ideally in two doses," said allergist Dr. Punita Ponda.

"These barriers include cost, inconvenience, lack of perception of need of the medication, poor knowledge of appropriate use," said Ponda, who helps direct allergy and immunology at Northwell Health, in Great Neck, N.Y.

The cost of the best-known auto-injector, EpiPen, made headlines in recent years as price tags rose to more than $600 by 2016. Public outcry ensued, and in December manufacturer Mylan announced it was producing a generic version for $300.

Dr. Nicole Berwald sees many cases of life-threatening allergies in her role as interim chair of emergency medicine at Staten Island University Hospital, in New York City.

She said, "Patients have reported expense -- either related to insurance co-pays, high deductibles or simply not having insurance coverage -- as a barrier to care."

But, she noted, other factors keep people from having the devices close at hand.

There are different brands of auto-injectors and "a patient's familiarity with one may not be the same as with another," Berwald said.

"In addition, many patients do not need to use their auto-injector frequently, and as a result lose their dexterity with the device," she explained. "Intermittent use may also explain why many people fail to keep the device on them at all times, which can have catastrophic consequences."

Many people also mistakenly believe that an auto-injector might be available for emergency use in many public spaces, according to Berwald.

Finally, she said, people who have so far only had a mild allergic reaction might think they are not at risk for a potentially fatal one.

However, "a subsequent reaction can be more severe, develop more rapidly, and be potentially life-threatening," Berwald said. "Without understanding this, individuals may not obtain the medication, or they may not reliably carry the medication with them."